This invention relates to controlled active switching.
As illustrated in FIG. 1, an ideal rectifier diode 1 is a two terminal device which conducts current, IR, of one polarity with essentially no loss (i.e., with no voltage drop, VR, across the rectifier) and which blocks current (and voltage) of the opposite polarity. In practice, rectifiers exhibit a forward voltage drop, VR=VF, when carrying current and conduct some reverse xe2x80x9cleakagexe2x80x9d current when blocking. The forward voltage drop results in power loss when the rectifier is conducting current.
The power loss associated with rectifier forward voltage drop in power supplies can be a significant source of loss. For example, FIG. 2 shows a schematic of a pulse-width-modulated (xe2x80x9cPWMxe2x80x9d) switching power supply 1 in which one or more switches (e.g., switches 3) are opened and closed to effect a transfer of energy from an input source 2 to a current-sinking load (as modeled by current source 7). The switching power supply may include one or more transformers 4 to provide isolation and voltage multiplication. In general, a pulsating voltage waveform, Vs(t), is delivered to the anode of forward rectifier 5. The forward rectifier 5 conducts current IF during some or all of the time that voltage Vs is positive (depending on the specific converter topology) and blocks during the remainder of the converter operating cycle. A xe2x80x9cfreewheelingxe2x80x9d rectifier 6 conducts current IS during the time that the forward rectifier 5 is blocking and blocks when the forward rectifier is conducting. Since each of the rectifiers conducts current during a portion of each operating cycle, power is dissipated in the rectifiers 5,6 throughout the entire operating cycle. Furthermore, since the average value of IL must be equal to the sum of the average values of IF and IS, and making the assumption that both of the rectifiers 5,6 have an essentially constant forward voltage drop, VF, when conducting, the forward loss in the rectifiers will be approximately equal to PFL=VF*IL, where IL is the average value of the load current. If the average value of the voltage across the load 7 is VO, then the power delivered to the load equals POUT=VO*IL. Thus, the loss in conversion efficiency owing solely to conduction losses in the rectifier diodes is approximately equal to: Rectifier Loss=100%*(PFL/POUT)=100%*(VF/VO).
The rectifier loss increases as the ratio of VF to VO increases. For example, if bipolar junction diode rectifiers (for which VF is approximately 0.7 volt) are used in a power supply having a 24 volt output they will result in a rectifier loss of approximately 100*(0.7/24)=2.9%. On the other hand, use of such diodes in a power supply delivering 3.3 volts would result in a rectifier loss of 21.2%. Using Schottky rectifiers (for which VF is approximately 0.4 volt) in the 3.3 volt power supply would result in a rectifier loss of 12.1%. In either instance, the amount of power dissipated in the rectifiers is substantial. In addition to conduction losses, rectifiers also exhibit switching losses associated with flow of reverse recovery currents during switching transitions. This can be particularly significant in bipolar junction diode rectifiers.
One way to reduce the efficiency loss, illustrated in FIGS. 3 and 4, is to use active switching devices, such as bipolar transistors 5a,6a or MOSFETs 5b,6b, in place of rectifier diodes. The forward switches 5a,5b are controlled to turn on during a forward conduction interval and to turn off during a blocking interval; the freewheeling switches 6a,6b are controlled to turn off during the forward conduction interval and to turn on during the blocking interval. Bipolar transistors can exhibit forward saturation voltage drops which are lower than the forward voltage drop of contemporary junction or Schottky diodes. MOSFETs with channel resistances of a few milliohms are also currently available. Thus, active switching devices can exhibit lower voltage drops, and rectification losses, than rectifier diodes used alone. In certain applications both of the rectifiers 5,6, FIG. 2, are replaced with active switching devices; in other applications only one of the rectifiers is replaced (depending on the relative average values of IF and IS); in yet other applications, rectifier diodes (e.g., diodes 5,6, FIG. 2) are bypassed with an active switching device. The process of controlling active switching devices to perform rectification is called xe2x80x9csynchronous rectification.xe2x80x9d Examples of synchronous rectification circuits are shown in Wymlenberg, U. S. Pat. No. 5,523,940; Martinez, U.S. Pat. No. 5,818,704; Rozman, U.S. Pat. No. 6,002,597; Yamashita, U.S. Pat. No. 5,726,869; Pasciutti, U.S. Pat. No. 3,663,941; Novac, U.S. Pat. No. 5,991,182; White, U.S. Pat. No. 4,870,555; Kolluri, U.S. Pat. No. 5,721,483; Shinada, U.S. Pat. No. 5,708,571; and in Patel, U.S. Pat. No. 4,716,514.
In general, in one aspect, the invention features a switching circuit having an active switch, a controller, and at least two terminals. The at least two terminals include two current control terminals for connection at two locations in another circuit. The controller is configured to turn the active switch off to block current between the two locations when the voltage between the two locations is of a first polarity and otherwise to turn the active switch on to conduct current between the two locations, whether or not the two current control terminals are the only ones of the at least two terminals that are connected to the other circuit.
Implementations of the invention may include one or more of the following features. There are exactly two terminals. The active switch includes a parallel diode. The active switch is a MOSFET. A bias subcircuit is configured to use power from the other circuit to provide a bias operating power to the switching circuit. The bias subcircuit includes a capacitor, a switch, and a bias voltage controller for the switch. The other circuit includes a power converter. The controller is configured to sense a polarity of the voltage between the two locations and to turn the active switch on and off in response to the sensed polarity.
In general, in another aspect, the invention features a method in which an active switch is turned off to block current between two locations in a circuit when the voltage between the two locations is of a first polarity and otherwise is turned on to conduct current between the two locations, without regard to voltages or currents at any other locations in the circuit.
In general, in another aspect, the invention features a method in which an end of each of two conductive sheets is attached to a respective one of at least two current carrying terminations of a semiconductor die. Another end of each of said two conductive sheets is attached to a respective one of two generally flat connection surfaces of a circuit component so that said switching element is in close proximity to an outer surface of said circuit component.
In general in another aspect the invention features apparatus having two conductive sheets, an end of each of the conductive sheets being attached to a respective one of at least two current carrying terminations of a semiconductor die. Another end of each of the conductive sheets is attached to a respective one of two generally flat connection surfaces of a circuit component, the connections being arranged so that the switching element is in close proximity to an outer surface of the circuit component.
Implementations of the invention may include one or more of the following features. The circuit component includes a capacitor of a power converter and the apparatus includes a two-terminal synchronous rectifier. The semiconductor die has circuitry for controlling the conductivity state of a MOSFET that is part of the die. The semiconductor die also has bias circuitry for generating a source of bias voltage.
In general, in another aspect, the invention features apparatus having a substrate and a semiconductor die that includes a controlled switching element and current carrying terminations. A conductive sheet has an end connected to a surface of the substrate and to one of the current-carrying terminations. A conductive strap has an end connected to another of the current-carrying terminations. Control circuitry controls a conductivity state of the switching element. Bias circuitry is configured to use power from the other circuit to provide operating power to the switching circuit.
Implementations of the invention may include one or more of the following features. The bias circuitry includes a storage capacitor. The control circuitry, the storage capacitor, and the bias circuitry are mounted to a surface of the substrate. The control circuitry and the bias circuitry are mounted to a surface of the substrate other than the surface to which the conductive sheet is connected. The semiconductor die includes the control circuitry and the bias circuitry. The current-carrying terminations lie on opposite parallel surfaces of the die. The end of the conductive strap is arranged in parallel with the opposite parallel surfaces and with the surface of the substrate. The apparatus includes multiple switching elements and the number of switching elements which are turned on is determined by the control circuitry based upon an amount of current being conducted. The switching devices are MOSFETs and the control circuitry makes the determination by measuring a voltage across the two locations when the controlled switches are turned on.
Other advantages and features will become apparent from the following description and from the claims.